Method for producing puzzolanic binders for the cement industry from steel slags using a reduction metal bath

ABSTRACT

In a method for producing puzzolanic or hydraulic grinding additives for the cement industry from basic oxidic slags, in particular steel slags, using a metal bath for the reduction of metal oxides contained in the slags, the basicities of the liquid slags are adjusted to values lying 0.1 to 0.5 below the basicity value (CaO/SiO 2 ) of the target slag prior to the reduction by adding acidic corrective substances such as, e.g., quartz sand and/or blast furnace slag and/or SiO 2 -containing corrective substances.

The invention relates to a method for producing puzzolanic or hydraulicgrinding additives for the cement industry from basic oxidic slags.

From EP 666 930 B1, a method has already been known, in which steelslags are reduced using pig iron and, in particular, the carbon content.present in the pig iron, thus causing, on the one hand, the pig ironbath to be refined and, at the same time, the iron oxide from the steelslag to be reduced to iron and get into the metal bath. Oxidic slagsand, in particular steelworks slags, depending on their metal oxidecontents and their basicities, have a more or less high viscosity, thuscalling for an operation at comparatively high temperatures in order tomaintain an accordingly highly liquid slag. If such slags are, moreover,adapted in view of cement-technologically interesting compositions bysuitable corrective substances, this will frequently result in slagstending to intensified foaming at usual temperatures on account of theintense CO formation from the carbon of the bath. If foams occurring inthat manner are stable, this will give rise to a reduced mass transferbetween metal and slag and hence a clearly lowered reduction rate, thussubstantially extending the treatment time. If, at the same time, it isoperated with a relatively high carbon content of the metal bath, thismay provoke particularly vigorous reactions in the interface, which maycause heavy foaming and even undesired slag spittings.

The invention aims to lower the initial reaction rate and shorten thetreatment time in the processing of such basic oxidic slags while, atthe same time, ensuring the safe and rapid reaction and, in particular,reduction of metal oxides in the slag at a low temperature level. Tosolve this object, the method according to the invention essentiallyconsists in that the basicities of the liquid slags are adjusted tovalues lying 0.1 to 0.5 below the basicity value (CaO/SiO₂) of thetarget slag prior to said reduction by adding acidic additives such as,e.g., quartz sand and/or blast furnace slag and/or SiO₂-containingcorrective substances. By lowering the basicities of the liquid slags torelatively low values and, in particular, lower values than appeardesirable for the subsequent utilization in cement technology, a slagthat is highly liquid even at lower temperatures and exhibits only aslight tendency to foaming can be immediately obtained. The decrease ofthe basicity and the thus connected lowering of the viscosity as afunction of the temperature level lead to a clearly higher reductionrate and hence a more rapid reaction during which, in particular, ironand manganese oxides are rapidly reduced to metallic iron and metallicmanganese, respectively. Likewise, oxides of chromium, nickel, vanadiumand other metals are reduced. With the metal oxide content decreasing,also the conversion of the carbon contained in the bath to carbonmonoxide and carbon dioxide naturally goes down, thus substantiallyreducing the risk of slag foaming. If appropriate amounts of Al₂O₃ areadded, the viscosity is additionally lowered, and a smaller decrease ofthe basicity will do in order to obtain the desired reaction rate.

Within the context of the method according to the invention, it issubsequently proceeded in an advantageous manner in that the basicitiesof the slags at the end, or near the end, of the reduction phase areadjusted to the desired target basicity of between 1.1. and 1.5. Onaccount of the diminished carbon conversion and the thus declinedtendency to foaming, it is, therefore, feasible to adjust the targetbasicity sought for cement-technological reasons already towards the endof the reduction, it being feasible to effect the respective addition inthe converter in which the reduction of the steel slags was performed.

In order to adjust a preferred cement-technological composition of theslags, also an increase in the Al₂O₃ content is usually desirable inaddition to the adjustment of a target basicity to values of between1.1. and 1.5, wherein bauxite may, for instance, be added for thatpurpose. In order to ensure a rapid conversion in the reduction phase,it will be of advantage if also the Al₂O₃-containing correctivesubstances are added already at the beginning of treatment, it beingadvantageously proceeded such that Al₂O₃-containing correctivesubstances such as, e.g., bauxite at least partially are added prior tothe reduction of the slags. By adding a partial amount of the additivesrequired for the adjustment of the Al₂O₃-content prior to reduction,reliable blending within the slag at a simultaneous decrease of the slagviscosity is safeguarded during the reduction procedure, said partialamount preferably ranging between one half and three quarters of therequired addition of Al₂O₃-containing additives.

In a particularly simple manner, the target basicity may be adjusted byadding burnt lime and/or CaO-containing corrective substances.

The lowering of the basicity to values near the neutral point prior toreduction also brings about particular advantages in respect to theservice life of a nonbasic refractory lining.

Within the context of the method according to the invention, it isadvantageously proceeded such that the target basicity is approachedfrom a point of time at which the metal oxide content and, inparticular, the sum of the iron oxide, manganese oxide, chromium oxide,nickel oxide and vanadium oxide contents of the slag falls below 3.5wt.-%. From a decrease of the metal oxide content to values of below 3.5wt.-%, foaming is already. largely avoided because of the substantiallydiminished conversion of the carbon contained in the bath as pointed outabove, so that it is already from that point of time that the additivesrequired for approaching the target basicity and thecement-technological composition may be added without adverselyaffecting the method.

In the following, the invention will be explained in more detail by wayof an exemplary embodiment.

EXAMPLE 1

In a converter, 3 tons of a slag having the composition indicated belowwere charged onto 10 tons of pig iron:

Slag wt. % CaO 45.1 SiO₂ 15.6 Al₂O₃ 3.3 MgO 6.7 TiO₂ 1.1 FeO 25.7 MnO2.5 CaO/SiO₂ 2.9

During the reduction procedure, carbon was supplied to the metal bath inthe form of carbon carriers. By feeding coal and oxygen, the temperaturerequired for maintaining a liquid slag was safeguarded and the necessaryreduction potential was provided. The slag basicity was lowered to avalue of 1.1 by blowing in 770 kg quartz sand, and 494 kg bauxite wereadded, whereby a slag having the following composition was formed at thebeginning of the melt reduction:

Slag composition prior to melt reduction wt. % CaO 35.5 SiO₂ 32.2 Al₂O₃9.4 MgO 5.5 TiO₂ 0.4 FeO 15.4 MnO 1.6 CaO/SiO₂ 1.1

Towards the end of the reduction phase in which, in particular, the FeOcontent could be lowered from 15.4 wt.-% to values of below 1% and themanganese oxide content could be substantially halved, the final slagcomposition sought was adjusted by the addition of 265 kg burnt lime and330 kg bauxite. The altogether relatively low metal oxide content ofabout 1.7 wt.-% and the slight conversion of the carbon contained in thebath to carbon monoxide and carbon dioxide towards the end of thereduction phase prevents foaming at that point of time till the end oftreatment. The following composition of the target slag, which stood outfor its excellent cement-technological properties, could be reached:

Slag composition after melt reduction wt. % CaO 44.7 SiO₂ 34.5 Al₂O₃14.5 MgO 4.3 TiO₂ 0.3 FeO 0.8 MnO 0.9 CaO/SiO₂ 1.3

Within the context of this method, the slag basicity was, thus, reducedto below the value of the target basicity by a total of 0.2, wherebyfoaming and undesired slag spitting could be safely prevented.

EXAMPLE 2

In a converter, 4 tons of a slag having the composition indicated belowwere charged in the liquid state onto 10 tons of pig iron:

Slag wt. % CaO 47.8 SiO₂ 26.3 Al₂O₃ 5.9 MgO 8.9 TiO₂ 1.3 FeO 1.7 MnO 1.4Cr₂O₃ 6.7 CaO/SiO₂ 1.8

The above-mentioned slag was subjected to a melt reduction process byfeeding coal and oxygen.

Yet, prior to that, the chemistry of the slag was changed in a mannerthat its viscosity decreased. This was effected by the addition of 867kg quartz sand and 980 kg bauxite.

At the same time, a reduction reaction set in already by themetal-oxide-containing slag contacting the carbon-containing iron bath.

Both effects (the adjustment of the basicity and Al₂O₃ content as wellas the commencing reduction) resulted in the following composition priorto the onset of the melt reduction treatment proper:

Slag composition prior to melt reduction wt. % CaO 35.6 SiO₂ 35.8 Al₂O₃14.1 MgO 6.7 TiO₂ 1.4 FeO 1.3 MnO 1.0 Cr₂O₃ 4.1 CaO/SiO₂ 1.0

After the end of the reduction phase, the final slag composition soughtwas adjusted by the addition of 828 kg burnt lime and 237 kg bauxite.

The composition of the final slag was determined as follows:

Slag composition after melt reduction wt. % CaO 45.0 SiO₂ 31.4 Al₂O₃14.4 MgO 5.9 TiO₂ 1.3 FeO 1.1 MnO 0.8 Cr₂O₃ 0.03 CaO/SiO₂ 1.4

What is claimed is:
 1. A method for producing additives for the cementindustry from basic oxidic slags, comprising the steps of chargingliquid slags onto a metal bath; adding at least one acidic substance tosaid liquid slags to adjust the basicity of said liquid slags to a value0.1 to 0.5 below a target basicity value (CaO/SiO₂) selected for targetslag; and reducing metal oxides in said liquid slags to produce thetarget slag.
 2. A method according to claim 1, wherein said at least oneacidic substance is at least one of the group consisting of quartz sand,blast furnace slag, and SiO₂-containing substances.
 3. A methodaccording to claim 1, wherein, at or near the end of said reduction ofmetal oxides, said target basicity value for said target slag is a valueof 1.1 to 1.5.
 4. A method according to claim 2, wherein, at or near theend of said reduction of metal oxides, said target basicity value forsaid target slag is a value of 1.1 to 1.5.
 5. A method according toclaim 1, wherein, Al₂O₃-containing substances are added to said liquidslags, and wherein at least a part of said Al₂O₃-containing substancesis added prior to said reduction of metal oxides.
 6. A method accordingto claim 2, wherein, Al₂O₃-containing substances are added to saidliquid slags, and wherein at least a part of said Al₂O₃-containingsubstances is added prior to said reduction of metal oxides.
 7. A methodaccording to claim 3, wherein, Al₂O₃-containing substances are added tosaid liquid slags, and wherein at least a part of said Al₂O₃-containingsubstances is added prior to said reduction of metal oxides.
 8. A methodaccording to claim 4, wherein, Al₂O₃-containing substances are added tosaid liquid slags, and wherein at least a part of said Al₂O₃-containingsubstances is added prior to said reduction of metal oxides.
 9. A methodaccording to claim 5, wherein said Al₂O₃-containing substance isbauxite.
 10. A method according to claim 1, wherein said target basicityvalue for said target slag is adjusted by adding at least one of thegroup consisting of burnt lime and CaO-containing substances.
 11. Amethod according to claim 2, wherein said target basicity value for saidtarget slag is adjusted by adding at least one of the group consistingof burnt lime and CaO-containing substances.
 12. A method according toclaim 3, wherein said target basicity value for said target slag isadjusted by adding at least one of the group consisting of burnt limeand CaO-containing substances.
 13. A method according to claim 4,wherein said target basicity value for said target slag is adjusted byadding at least one of the group consisting of burnt lime andCaO-containing substances.
 14. A method according to claim 5, whereinsaid target basicity value for said target slag is adjusted by adding atleast one of the group consisting of burnt lime CaO-containingsubstances.
 15. A method according to claim 1, wherein said targetbasicity value is approached at the point in time at which the metaloxide content of the liquid slag falls below 3.5 percent by weight. 16.A method according to claim 2, wherein said target basicity value isapproached at the point in time at which the metal oxide content of theliquid slag falls below 3.5 percent by weight.
 17. A method according toclaim 3, wherein said target basicity value is approached at the pointin time at which the metal oxide content of the liquid slag falls below3.5 percent by weight.
 18. A method according to claim 4, wherein saidtarget basicity value is approached at the point in time at which themetal oxide content of the liquid slag falls below 3.5 percent byweight.
 19. A method according to claim 5, wherein the Al₂O₃-containingsubstances are added in an amount aimed to obtain an Al₂O₃-content of 12to 15 percent by weight, whereby basicity is lowered by a slighterextent than at lower Al₂O₃ contents.
 20. A method according to claim 6,wherein the Al₂O₃-containing substances are added in an amount aimed toobtain an Al₂O₃-content of 12 to 15 percent by weight, whereby basicityis lowered by a slighter extent than at lower Al₂O₃ contents.